1. Field of the Invention
The present invention relates to a method of and an apparatus for controlling writing power in an optical drive, and more particularly, to a method of and an apparatus for controlling power for driving a laser diode (LD) in a write mode, thereby obtaining an optimum writing characteristic where a writing environment changes.
2. Description of the Related Art
Conventional optical drives control a level of laser power (or writing power) necessary for writing data to an optical medium such as a Compact Disc Recordable (CD-R) or a Compact Disc ReWritable (CD-RW), to which data may be additionally written or rewritten, using a power calibration area (PCA) on the optical medium in a write mode. This control is referred to as an optimum power control (OPC).
Even where optimum writing power is obtained due to OPC, a satisfactory writing characteristic is not attained where a writing environment changes in an actual write mode. For example, where fluctuation in power sensitivity occurs between different positions on an optical medium, where a wavelength of an LD shifts due to a high temperature, where a deviation occurs in a beam spot due to disc skew, the thickness of a disc or defocus, or where disc and/or optical conditions change because actual writing is performed long after OPC is performed, optimum writing power determined by the OPC changes, thereby modifying the beta or asymmetry of a written signal. The term beta describes asymmetry between a top level and a bottom level of an AC-coupled RF signal. Consequently, a satisfactory writing characteristic is not attained.
To overcome this problem, running OPC has been proposed to control the writing power of an LD such that an optimum writing characteristic is attained even where a writing environment changes in the course of writing data. According to conventional OPC, in a case where a write pulse incident on an optical medium is generated, as shown as writing power versus time in FIG. 1A, a level of a write pulse reflected from the optical medium decreases at a data recordable side of the optical medium when a pit starts to be formed on the optical medium, as shown in FIG. 1B. As shown in FIG. 1A, a peak level of an incident write pulse increases where incident writing power increases from a level 1 to a level 6. As shown in FIG. 1B, as the incident writing power increases from the level 1 to a level 3, the peak level of a reflected write pulse also increases. However, the peak level of a reflected write pulse rapidly drops and then levels off so that the reflected write pulse has a nonlinear characteristic at each of the fourth through sixth levels of the incident writing power, since reflectance decreases by forming a pit on a data recordable side. The reflected writing pulse is sampled when the peak level of the reflected write pulse is constant, as shown in FIG. 1B.
According to a relation between the incident write pulse and the reflected write pulse, a radio frequency (RF) signal (referred to as a WRF_SH), which is obtained by sampling and holding a signal (referred to as a WRF) corresponding to the quantity of light reflected by the optical medium, has a nonlinear characteristic with respect to the writing power, as illustrated by Δ in FIG. 2. Accordingly, the conventional running OPC is realized such that WRF_SH is divided by a signal (referred to as a MPDO_SH), which is obtained by sampling and holding the output signal (referred to as a MPDO) from a monitoring photo diode, to obtain a normalized write RF signal (referred to as a WRF_Norm) having a linear characteristic with respect to writing power, as illustrated by □ in FIG. 2, according to Equation (1), thereby controlling the writing power of an LD.                     WRF_Norm        =                  WRF_SH          MPDO_SH                                    (        1        )            
However, since values of the WRF and the MPDO are not suitable for obtaining a sufficient dynamic range in hardware, WRF_Norm obtained using WRF_SH and MPDO_SH has a nonlinear characteristic, as shown in FIG. 2. Accordingly, when skew exists due to an optical medium or a deck mechanism, where the wavelength of the LD changes due to high temperature, or where deviation occurs in an optical medium, control of the writing power of the LD to be in an optimum state becomes impossible. As a result, a difference between a beta of a signal written to an inner track and a beta of a signal written to an outer track on an optical medium increases, and a pit deviation between the inner track and the outer track also increases. As such, a difference in a land jitter between the inner track and the outer track increases so that it is difficult to attain a satisfactory writing characteristic.